1. Field of the Invention
The present invention relates to a CMOS image sensor and other photo detecting devices.
2. Description of the Related Art
In recent years, CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) image sensors are often used in a variety of image pickup apparatuses, such as digital still cameras and digital video cameras.
A number of benefits can be gained by choosing a CMOS image sensor. For example, a CMOS image sensor can be produced on the same manufacturing line as general chips, and it can be turned into a single chip together with the peripheral functions. Also, a CMOS image sensor, which can be driven at lower voltage than a CCD type, consumes less power than it.
Each pixel of a CMOS sensor has a structure including a photodiode and a switch using MOSFETs. That is, the sensor has a matrix of photodiodes, each of which has a switch attached thereto, and the potential of each pixel is read out by operating these switches one by one.
FIG. 1 is a circuit diagram showing a structure of a pixel circuit 200 of a conventional CMOS image sensor. This pixel circuit 200 includes a photodiode PD, a reset transistor M11, an amplifying transistor M12, and an output transistor M13. The reset transistor M11, the amplifying transistor M12, and the output transistor M13 are each an n-channel MOSFET. The reset transistor M11 and the photodiode PD are connected in series between a supply voltage Vdd and a ground voltage GND. A source terminal of the reset transistor M11 is connected to the photodiode PD, and the supply voltage Vdd is applied to a drain terminal thereof. A reset signal RST is inputted to a gate terminal of the reset transistor M11.
A cathode terminal of the photodiode PD, which is connected with the reset transistor M11, is connected to a gate terminal of the amplifying transistor M12. When the supply voltage Vdd is applied to a drain terminal of the amplifying transistor M12 and a source terminal is connected to a drain terminal of the transistor M13, the amplifying transistor M12 functions as a source follower. A source terminal of the output transistor M13 is connected to a data line LD, which is provided for each column of the CMOS image sensor.
In a pixel circuit 200 structured as described above, when a reset signal RST inputted to the gate terminal of the reset transistor M11 goes to a high level, the reset transistor M11 turns on, thereby applying a supply voltage Vdd to the photodiode PD and charging the cathode terminal thereof with the supply voltage Vdd. Next, the reset transistor M11 turns off. In this state, if light strikes the photodiode PD, a photocurrent will flow, and a negative charge will be stored in the cathode terminal of the photodiode PD. At this time, the voltage at the cathode terminal of the photodiode PD changes with the light intensity and the charge storage time.
After the passage of a predetermined storage time, a selection signal SEL is set to a high level, which turns on the output transistor M13. As a result, the voltage at the cathode terminal of the photodiode PD is amplified by the amplifying transistor M12 and outputted to the data line LD. In this manner, a voltage corresponding to the amount of light received by the photodiode is outputted to the data line LD, and an external circuit can read the amount of light received by each pixel circuit.
The following discussion concerns the dynamic range of a pixel circuit of a conventional CMOS sensor as shown in FIG. 1. As described above, in detecting the amount of light received by each pixel, the photodiode PD is charged with a power supply voltage Vdd, a negative charge is stored at the cathode terminal of the photodiode PD during the exposure period, and the charge amount is converted into voltage to determine the amount of light received. Accordingly, if a strong light enters the photodiode PD and the voltage at the cathode terminal of the photodiode PD drops significantly during the charge storage period, then the amplifying transistor M12 will stop amplifying the voltage at the cathode terminal of the photodiode PD. Consequently, the pixel circuit 200 can no longer detect the amount of light having entered the photodiode PD.
Conversely, if the charge storage time is shortened, the voltage at the cathode terminal of the photodiode PD will not drop much, so that strong light may be detected. However, if a weak light enters in this state, then the voltage at the cathode terminal of the photodiode PD will little change, so that the weak incident light may not be detected. With the conventional pixel circuit 200, therefore, the dynamic range is subject to limitation by the amount of initial charge stored at the cathode terminal of the photodiode PD in the reset state. And the conventional technique for widening the dynamic range has been through logarithmic conversion or changing the charge storage time and gain.
Such techniques employ a form of circuit called an active pixel sensor as shown in FIG. 1. This, however, has a problem that the shorter the charge storage time is made, the more the power consumption will be for driving the circuit at high speed. Also, in changing the gain, it is inevitable that the circuit be made larger in scale if the gain is to be set high.